Gas turbine engines generally have a plurality of axially aligned components including a fan, a compressor section, a combustor, and a turbine section. The fan, positioned at a forward end of the engine, rotates to draw in and pressurize ambient air. Some of the accelerated air flows to the compressor section, as a core flow, where the air is compressed and then flows to the combustor. At the combustor, the compressed air is mixed with fuel and combusted to form an exhaust. The exhaust expands from the combustor through the turbine section, causing turbines of the turbine section to rotate, and then flowing out of the engine at an aft end of the engine. The rotation of the turbines drives the rotation of the fan and compressors by way of a shaft, or a plurality of concentrically mounted shafts in the case of a multi-spool engine. It can therefore be seen that once this process has begun it is self sustaining.
A nacelle surrounds the engine and includes an inlet forward of the fan. The fan typically includes a central hub, or rotor disk, and a plurality of blades radially extending outward from the hub. When the fan rotates at a sufficiently high speed, the most radially distant extent of the blades, a tip of each blade, travels at supersonic speeds and generates an aerodynamic shock that generally travels in the forward direction.
Since the blades may possesses slight, but acceptable, structural and alignment differences incurred during manufacture and installation, the shock produced by each blade may travel at different speeds and/or with different orientations. These different shocks interact with each other and the inlet to produce a time varying air pressure pattern that repeats upon the completion of each rotation of the fan, also known as combination tone noise, multiple pure tone noise, or “buzz saw” noise. This combination tone noise travels in the forward direction out of the inlet and may be undesirable to people in a cabin of an aircraft utilizing the engine and in a community around the aircraft.
Many methods of reducing the noise generated by the engine in this fashion have been developed. One such method may include positioning the blades of the fan in an arrangement to force decay-prone harmonics to be generated by the fan. These decay-prone harmonics then decay in the inlet as described in the U.S. Pat. No. 5,966,525. Another method may be to actively shift all harmonics to higher orders and then attenuate the harmonics with an acoustic liner, such as in the U.S. Pat. No. 5,979,593.
While effective, new engine designs may generate decay-prone harmonics too closely to a lip of the inlet of the nacelle to effectively decay in the inlet. Further, active methods of noise reduction require moving components and/or electricity. Such components are maintenance intensive and any electricity usage drains output from the engine. Therefore, a new passive method for reducing the noise generated by the fan is needed.